1. Field of the Invention
This invention relates in general to earth boring equipment and in particular to a stabilizer for centralizing the drill string in shaft boring.
2. Description of the Prior Art
In the drilling of large diameter wells or shafts, which may be four feet to twenty feet or so in diameter, it is important to center the drill string in the shaft, the typical drill pipe being only about 16 inches in diameter. One prior stabilizer has a frame extending outward from the pipe. A number of vertical cylinders or rollers are mounted on bearings at the edge of the frame. The frame rotates in unison with the drill string, and the rollers engage the shaft wall in rolling contact.
One disadvantage of this type of stabilizer is that it is expensive to construct. Also the bearings in the rollers require maintenance. Another disadvantage is in disassembling the stabilizer. When pulling the string from the well, frequently the opening in the rig floor is of lesser diameter than the drill bit. There is sufficient clearance between the rig floor and the earth's surface to remove the bit from the shaft and perform the necessary operations on it below the rig floor. The stabilizer, which is normally several feet above the drill bit, however, must be first disassembled since there is normally not enough clearance for both the assembled stabilizer and bit to be drawn into the space beneath the rig floor.